1. Field of the Invention
The interest for proton conducting polymeric membranes has considerably grown in recent years for their possible use in fuel cells of the PEM FC type (Polymeric Electrolyte Membrane Fuel Cells) that, as well known, are presently the most suitable power supplies for electrical cars or for electrical portable devices such as computers, cellular telephones, telecameras etc. Owing to their strong acid and sometime super acid properties, these proton conducting membranes can also find applications in membrane catalytic reactors, especially for acid, catalysed processes.
Nafion membranes, owing to their excellent chemical stability and high proton conductivity, are presently the most suitable membranes for PEM FC using pure hydrogen as fuel. However, Nafion membranes are very expensive, loss efficiency at temperature higher than 80-9020 C. and besides are permeable to methanol.
2. Description of Related Art
This leads to an increase of the already high costs of PEM FC, complicate the cooling of the stacks and does not allow their use in PEM FCs using methanol as fuel. On the other hand, the well-known contamination of the air, provoked, especially in large towns, by the intense vehicular traffic, makes the demand of less pollutant innovative cars more and more urgent. Among the different possible solutions offered by today's technology, presently the preference seems to be oriented towards electrical vehicles fed by PEM FCs. Thus, in the most industrialized countries, attempts are in progress to prepare new proton conducting membranes less expensive and exhibiting better characteristics than Nafion as far as a higher working temperature and a lower permeability to methanol are concerned. All this, in the hope that more economical and more efficient PEM FC may be obtained with these membranes.
Two main research lines are presently followed: a) development of new ionomeric membranes with the above-mentioned characteristics; b) development of membranes prepared with ionomers of the state of art but with improved characteristics.
Concerning the latter point, the improvement of the characteristics of an ionomer can be obtained by taking advantage of the present knowledge on polymeric nano-composites. A polymeric nano-composite (or nano-polymer) is a polymer, which contains an appreciable amount of inorganic nano-particles dispersed as uniformly as possible within its polymeric matrix. Due to the high specific surface of the nano-particles, their interfacial interaction with the polymer matrix may become very strong and may therefore have a deep influence on the properties of the polymer itself. The stronger the interfacial interaction, the greater the influence of the nano-particles on the polymer characteristics. In this respect, the high specific surface of lamellar nano-particles seems to be very convenient. The recent industrial success on polymeric nano-composites containing organophilic clays has clearly demonstrated that the presence of the dispersed lamellar nano-particles, obtained by exfoliation of clays, may indeed modify to a great extent some important properties of the polymers such as inflammability, permeability to neutral or ionic species, mechanical resistance and thermal stability.
The strategy of dispersing fine powders of inorganic compounds such as silica and zirconium phosphate in ionomers of the state of art has been seldom used but it is not new. For example, in the U.S. Pat. No. 5,523,181 it is claimed that silica gel particles dispersed in ionomers favours the maintenance of the water balance, thus allowing the fuel cell to work with reduced humidity in the flux of gaseous reagents. In the Italian patent IT MI 002443 (14 Nov. 2000) has been reported that the presence of silica gel nano-particles or zirconium phosphonate particles in the electrode/membrane interface regions can appreciably improve the performance of hydrogen PEM FC at temperatures >80° C. In the international patent WO 96/29752 of DU PONT DE NEMOURS AND COMPANY (Us/Us) it is claimed that lamellar zirconium acid phosphate too may modify some properties of Nafion, improving for example the mechanical properties or reducing its permeability to methanol. However, it must be pointed out that the addition of non-conducting particles, such as silica, or of particles with low proton conductivity, such as zirconium phosphate, must be limited (usually less than 10 wt %) because the addition of larger amounts could reduce the proton conductivity of the ionomers thus lowering the electrical performance of the fuel cell. Moreover, it must be pointed out that the influence of the dimension and orientation of the particles on the above properties was not well understood and therefore not reported in said international patent. Furthermore, since zirconium phosphate is completely insoluble in known solvents, the insertion has been performed by “in situ precipitation”. This method does not give any guarantee either concerning the dimensions and exfoliation degree of the formed precipitates or the orientation of the particles.